Accurately measuring the elevated temperature of a remote object in cold wall non-equilibrium furnaces, using optical techniques and without touching the object is complicated because of undesired reflections of ambient radiation from the heat source along with the desired radiation from the heated object itself. Optical pyrometry allows the temperature of an object to be measured remotely by analyzing the radiation emitted by the object. Of course, all objects at temperatures greater than zero Kelvin emit radiation which can be measured to determine the temperature of the object, provided the emissivity of the object is known. Thus, optical pyrometry operates upon the underlying principle that as the temperature of an object increases, the radiation it emits shifts in wavelength and increases in intensity so that an object which emits radiation with an orange glow is hotter than an otherwise identical object which emits radiation with a red glow. Such temperature-measuring schemes are discussed in the literature (see, for example, Tenney; Mechanical Engineering, Oct. 1986; "Red Hot . . . AND HOTTER, " pp. 36-41).
Certain processes for fabricating circuits on silicon wafers require accurate measurement from a remote location of the temperature of a wafer within a processing furnace. This measurement is complicated because in almost all cases the wafer emissivity is unknown. The emissivity of the wafer is altered by different treatments to its backside surface or by additional back side coatings on the wafer of different thicknesses and different materials such as silicon dioxide or silicon nitride, polysilicon or metals. Optical temperature measurements of the wafer must therefore be corrected for emissivity of the wafer to provide accurate temperature measurements of the wafer. Since the radiation emitted from the wafer and sensed by the detector depends both on its temperature and its emissivity, a single measurement at one specific wavelength is insufficient to determine wafer temperature unless its emissivity is known. For a single-wavelength pyrometer to be useful, wafer emissivity is first measured, for example, by apparatus of the type disclosed in U.S. Pat. No. 4,854,727 entitled "Emissivity Calibration Apparatus and Method." Once emissivity is known, it can be used to determine wafer temperature from a single wavelength pyrometer measurement. However, since the determination of emissivity using a thermocouple reference is, in most cases, a destructive and lengthy process, it cannot be used on production wafers which are to be heat treated. One scheme used in such limiting cases is to measure the emissivity of one typical wafer from a batch, designated as the Master Wafer. Assuming that emissivities of the remaining wafers in the batch are close or identical to the emissivity of the Master Wafer, then this value of emissivity can be used to translate single wavelength pyrometer reading to true temperature of the wafer.
However, in many cases, the emissivities of wafers within a specific batch vary widely, and use of a single emissivity value for the entire batch of wafers will cause significant variations in measured wafer temperatures with potentially detrimental consequences in the processing of the wafers. The accuracy of conventional methods for optically measuring wafer temperatures is usually not adequate for temperature-controlling applications. Thus, it would be desirable to perform non-destructive, non-contacting and fast measurements on each wafer to concurrently determine its emissivity and temperature to necessary accuracies for acceptable batch process control in order to obviate the effects of reasonable changes in emissivity from wafer to wafer within a batch.
Wafer radiation may be conventionally measured in two or more wavebands to supply enough information from which to determine wafer emissivity and temperature. However, these measurement techniques generally rely on the fact that the effective emissivity of a wafer, or other object, is identical in each of the two or more measurement wavebands. On the assumption that the effective emissivity values are the same in different wavebands, they can be cancelled out in the determination of temperature by calculating a ratio of optical measurements. However, this assumption is not accurate or realistic for semiconductor wafers with diverse coatings on the back sides (from which the radiation is measured). In most cases, the emissivity of a semiconductor wafer is a function of both wavelength and temperature and therefore the above assumption in the prior art techniques contribute to erroneous temperature determinations by pyrometric measurement techniques.